Combining two data sources into one or making a parent and child relation with two different sources

Hello,

So just to set some context around this. We are trying to work around the limitation of returning only at most 10k results during a search and with filters due to having an immense amount of data and it also coming from multiple indices.

Could I join multiple indices with different fields into one? Sounds like a mess but I figured I'd ask if it's possible.

The second idea is instead of using the child/parent join with the documents if it could be used with two indices. Is this also feasible or is there some other solution.

Have you considered using search_after and just running several requests using Point in Time?

So due to time constraints and current setup on our spring boot side we were siding with using the Reindex API in conjunction with the parent-child join, however, the parent child join is a bit confusing. As in using the join_field or is that a said an actual field.

This is from an example I saw in a different post:
if(ctx._source.parent_id != null){ctx._routing = ctx._source.parent_id; ctx._source.join_field= params.cjoin}else{ctx._source.join_field = params.parent_join}",
"params" : {
"cjoin" :{
"name": "child_message",
"parent": 1
},
"parent_join" : "parent_message"

}